Yes, it's usual.
The police only normally get involved when there is evidence of a criminal act.
Whilst the AAIB *may* conclude that there has been misjudgement or negligence, it is near certain that nobody intended this accident to happen or believed that it was likely. If the police have taken that view and to leave this to be treated as an accident for lessons to be learned then I, for one, would applaud that decision.
G